1. Field of the Invention
The present invention relates to a process for the manufacturing of a circuit board which interconnects at least two circuit patterns, a removable film suitable for a process for the manufacturing of the circuit board, a substrate with film and a process for forming the removable film.
2. Description of the Related Art
As electronic equipment, in recent years, downsize and become denser starting with information-telecommunication, a circuit board is deeply required to brought to the multilayer not only in the field of the industrial use but also in the field of the consumer use. It is required in multilayered circuit board to develop an interconnection structure contacting through interstitial via hole between a plurality of circuit patterns. Further, it is required to develop a structure and a process with a high degree of reliability in a structure of interstitial via hole contact and a process for preparing its structure.
The present applicant proposed a novelly configured circuit board interconnecting between layers through a interstitial via hole by conductive paste and a novel process for preparing the circuit board.
A process for the manufacturing of the circuit board is composed as follows.
That is, a process for the manufacturing of the circuit board is composed, comprising:
a procedure for pasting removable mask film to porous insulating substrate consisting of a composite material of nonwoven fabric and thermosetting resin and having compressibility and providing a through hole in the substrate;
a procedure for filling conductive paste into said through hole;
a procedure for removing said film from said substrate with film filled with conductive paste;
a procedure for bonding metal foil to a face removed of film of said substrate; and
a procedure for compressing said substrate bonded with said metal foil by heating and pressuring.
In such a process for the manufacturing of the circuit board, carbon dioxide gas laser is generally used in drilling porous insulating substrate including removable mask film for providing through hole.
However, carbon dioxide gas laser is not easy to focus and hard to reduce the focus spot since a wavelength of laser light is relatively as long as 10.6 xcexcm. Furthermore, since a wavelength of laser light exists in a range of infra-red rays, through hole is enlarged due to the thermal effects of laser. Therefore, it is difficult to form through holes, for example, as fine as the diameter of 50 xcexcm and this poses the barrier for realizing a circuit board of fine interconnection design.
Further, in a process for the manufacturing of a circuit board described above, a removable mask films are pasted to the porous insulating substrate by thermal laminating process. Further, hole drilling is performed on a porous insulating substrate with removable mask films by utilizing laser processing and the like. Furthermore, conductive paste is filled into the drilled holes and thereafter the removable the mask film is removed.
In such a process, it is in a condition that strain developed by heat, pressure and film tension in laminating remains in said substrate. When removable film is removed from said substrate under this condition, said residual strain is released from said film and drilled holes result in a deviation from the hole positions at drilling. This causes a significant problem particularly in the case required for narrow hole pitch and fine dimensional accuracy, that is, in realizing a circuit board of fine interconnection design.
Therefor, it is an principal object of the present invention to provide a process for the manufacturing of a circuit board which may realize a circuit board of fine interconnection design as well as to provide a removable film suitable for a process for the manufacturing of the circuit board, a substrate with film and a process for forming the removable film.
It is another object of the present invention to provide a circuit board which protects a drilled hole from being filled together with scraping and dust generated during drilling in filling conductive paste into the drilled hole and enables via hole contact of low initial resistance and high interconnection reliability.
To accomplish objectives described above, the present invention, in short, employs removable mask film absorbing laser light of wavelength not longer than that in the ultraviolet range. Thus, hole drilling may be performed by using laser light of wavelength not longer than that in the ultraviolet range in which laser beam is easy to focus more and fine hole drilling may be performed.
Further, said substrate is preferably a film substrate having optical absorptivity upon laser light of wavelength not longer than that in the ultraviolet range and adhesion or preferably a porous substrate having optical absorptivity upon laser light of wavelength not longer than that in the ultraviolet range and compressibility. Thereby, it becomes that fine hole drilling may be performed on a substrate and removable film at one time by using laser light of wavelength not longer than that in the ultraviolet range.
Further, a removable film preferably contains a ultraviolet-absorbing agent. Thereby, it becomes that a removable film may absorb laser light with a wavelength not longer than that in the ultraviolet range by addition of a ultraviolet-absorbing agent even though a principal material composing a removable film does not have a absorption characteristic of ultraviolet light.
Further, a removable film is preferably cellulosic, such as nitro cellulose, acethyl cellulose, cellulose acetate, cellulose propionate, ethyl cellulose and the like. Thereby, it becomes that a removable film may be formed by using cellulosic having the good ability of the film to be formed and the good solubility into the solvent.
Further, preferably, said removable film is formed by applying and drying a resin varnish, which exerts removability after drying and has optical absorptivity upon laser light in a range of wavelength not longer than that in the ultraviolet range, on the surface of the substrate. In conventional embodiments forming removable mask films on substrates by thermal laminating process, a residual strain develops in the substrate due to heat, pressure and film tension in laminating and the residual strain is thereafter released in removing the films to result in a dimensional deviation. On the contrary, in the present process using a resin varnish, such a dimensional deviation will be improved.
Further, a removable film is preferably polyethylene naphthalate (PEN), polyamide or polyimide.
Further, said resin varnish preferably contains a solvent not eroding said substrate. Thereby, the removable film will be able to be formed on the substrate without eroding the substrate.
Further, a plurality of removable films are preferably laminated and provided on a substrate. Thereby, a superficial removable film may be removed after hole drilling, and scraping generated in hole drilling and dust deposited during a processing may be removed together with a removable film removed. Thereby, a drilled hole is protected from being contaminated with scraping and dust during filling a conductive element into the drilled hole. Therefore, a circuit board having via hole contact of low initial resistance and high interconnection reliability may be obtained. Furthermore, since at least one removable film is left on the side of the substrate after a superficial removable film is removed, the removable film functions as a mask film in filling the conductive element and therefore a surface of a insulating resin sheet is not contaminated with the conductive element.